text: class Circle {
    constructor(center, radius) {
      this.center = center;
      this.radius = radius;
    }
    area() {
      return Math.PI*this.radius*this.radius;
    }
    toString() {
      return "Circle " + "중심정 (" + this.center.x + "," + this.center.y + "), 반지름 = " + this.radius;
    }
  }
  
  class Ball extends Circle {
    move(dx, dy) {
      this.center.x += dx;
      this.center.y += dy;
    }
  }
  
  var ball = new Ball({x: 0, y: 0}, 2);
  console.log(ball.toString());
  console.log(ball.area());
  ball.move(1, 2);
  console.log(ball.toString());
  
  // Circle 중심정 (0,0), 반지름 = 2
  // 12.566370614359172
  // Circle 중심정 (1,2), 반지름 = 2